Trim off any hanging bits of fat or meat from the rib roast. Apply 2 tablespoons of kosher salt evenly to roast. Place on a large platter and store in the refrigerator anywhere between 45 minutes to 12 hours, uncovered.
Soak wood chips in water for 1 hour.
Set up a two-zone fire on your kettle. That means banking all the hot coals on one side (direct heat zone), and leaving the other side empty (indirect heat zone). We will be cooking the rib roast over the indirect heat.Place a drip pan next to the coals to catch the drippings. Replace the grate before placing the roast on the grill. See step 5 for more details.
While grill is heating, prepare the butter-herb rub. Apply evenly to the pre salted rib roast. Let roast stand at room temperature for 15-20 minutes before placing on the grill.
Add pre soaked wood chips to grill according to manufacturer's recommendations. Set rib roast on grill grate. Set an aluminum drip pan filled with 1 cup of water directly underneath. Cook for 2 1/2-3 hours, adding fresh wood chips every 30 minutes. I recommend a doneness temperature of 130 degrees F or medium-rare.
Once cooked , remove rib roast from grill and place it onto a clean cutting board. Loosely tent roast with aluminum foil, and let the meat rest for 20-30 minutes before carving.
While roast is resting, prepare the sauce. Combine sour cream, 1/4 cup Dijon mustard, whole milk, green onion, and ranch dressing in a bowl. Serve on top of sliced rib roast or on the side. If you'd rather skip this step, then serve roast with gravy and horseradish sauce.